1. Technical Field of the Invention
The present invention relates to a power conversion device with synchronous rectifier driving and, in particular, but not exclusively, to the control of the synchronous rectifiers used inside a power conversion device or power unit in the switching mode.
2. Description of Related Art
As is well known in the field of power conversion devices, particularly low output voltage DC converters, the use of MOS transistors as rectifiers is an increasingly used technique because of its beneficial effect on circuit efficiency due to the low conduction losses of these devices with respect to the traditional rectifiers realized by means of Schottky diodes.
The way a synchronous rectifier (SR) is controlled is fundamental for the correct operation of the conversion device. Convenient techniques have thus been used to drive these synchronous rectifiers according to the law of the diode to be replaced by the synchronous rectifier. For example, by deriving a driving signal from a main control signal PWM it is possible to determine the different conditions of the conversion device in the switching mode and thus of the management conditions for the device diodes.
The way the driving signal can be derived from the main signal PWM, in order to correctly control a synchronous rectifier, depends on the kind of circuit topology used and on the galvanic insulation of this topology.
In particular, in an non-insulated topology of a conversion device or power unit in the switching mode, a synchronous rectifier control circuit can obtain information about the main switch switching transitions (turn-on and turn-off) from the main control circuit in a very simple way.
In insulated topologies, if the main signal PWM is located on the secondary side, the synchronous rectifier driving operation can be easily solved. In fact, since the signal PWM is available on the secondary side, it can be used to generate the synchronous rectifier driving signal. Convenient delays can be added to the signal PWM to compensate propagation delays affecting the driving signal being transferred to the primary side by means of a convenient coupling device. The timing required for this kind of operation is shown in FIG. 1A, in the more general case of two complementary signals on the secondary side.
It is worth remembering that it is necessary to provide some dead time between the driving signals to prevent a simultaneous conduction between synchronous rectifiers or between a synchronous rectifier and a main MOS transistor.
In particular, FIG. 1A indicates the timing required for a synchronous rectifier driving signal according to a general topology configuration in the switching mode with a main switch and only one diode, wherein the possible conduction times for the switch and the diode are complementary.
Dead times, as shown in FIG. 1A, prevent the simultaneous conduction of the main switch and of the synchronous rectifier used as diode, but they must be reduced to the lowest time value to minimize the parasitic diode conduction times of the synchronous rectifier and the subsequent efficiency loss.
On the other hand, in insulated topologies with main control on the primary side, the absence of the control signal PWM on the secondary side of the insulation barrier makes the generation of convenient control signals for the synchronous rectifiers more difficult. In this case, only the voltage signal on the transformer secondary side provides the information about the main switch switching transitions (turn-on and turn-off). However, this signal is difficult to manage, mainly because of the two following effects:
1. the delay introduced by the insulation transformer; and
2. the non-squared waveform during the discontinuous operation mode.
The problem related to the delay introduced by the insulation transformer has been faced and solved in European Application for Patent no. 1,148,624 filed in the name of the same Applicant, the disclosure of which is hereby incorporated by reference.
The problem related to the non-squared waveform is linked to the fact that the clock signal output by the insulation transformer is affected by the switching of the main switch.
In fact, this signal shows a similar behavior to the main signal PWM, at least in the continuous conduction mode, but it is also affected by the parasitic elements in the conversion device. Moreover, if the continuous operation state is not respected, the oscillations of this signal can determine false driving information.
Convenient synchronous rectifier driving signals must thus be provided, effective to prevent possible wrong active conditions derived from all the effects of timing on the synchronization of the signal available on the secondary clock signal PWM with respect to the primary signal PWM from occurring.
By using the output of the insulation transformer as the clock signal PWM, it is known to use a technique known as “self-driving synchronous rectification”, schematically shown in FIG. 1, and globally indicated with 1, to let MOS transistors operate like rectifiers, particularly in insulated topologies based on the forward topology.
The device 1 is connected to the winding 2 of a transformer and it comprises a first MOS transistor M2, connected between a first terminal T1 and a second terminal T2 of the winding 2 and having a gate terminal connected to the second terminal T2. The device 1 also comprises a second MOS transistor M1, connected between the second terminal T2 of the winding 2 and an output terminal OUT2 of the device 1 and having a gate terminal connected to the first terminal T1.
The device 1 also comprises an inductor L1 connected between the first terminal T1 and a further output terminal OUT1 of the device 1, as well as a capacitor C1, connected between the output terminals OUT1 and OUT2. The inductor L1 and the capacitor C1 form a linear output stage LC.
It should be noted that the connection of the output terminals of MOS transistors M1 and M2 to the terminals T1 and T2 allows these transistors to be correctly driven, according to the diode equivalent law.
In fact, the information related to the primary winding switching transitions are actually provided by the square component of the output signal of the winding 2.
In discontinuous device operation modes, when the current in the inductor L1 reaches the zero value, the oscillation of the reactive elements of the device 1 thus complicates the detection of the switching transitions on the winding 2.
It is thus known to equip the device 1 with a driving circuit 3 effective to drive synchronous rectifiers, indicated with SR1 and SR2 and realized by these MOS transistors M1 and M2, as schematically shown in FIG. 2. In particular, the device 1 shown in FIG. 2 is single-output-configured, i.e., it has the second terminal T2 connected to a potential reference, particularly to the ground GND, for convenience of illustration.
In particular, the driving circuit 3 must be able to manage a clock signal separately from the insulation transformer output and to solve any other problem concerning the signal synchronization on the output terminals OUT1, OUT2 of the device 1, particularly with respect to the clock signal.
Moreover, the driving circuit 3 must set the timing of the synchronous rectifier driving signal starting from the clock signal, providing, as already explained, convenient dead times between the clock signal and the driving signal of the synchronous rectifier RS to avoid the crossed conduction between the device elements.
The driving circuit 3 shown in FIG. 2 has a first input terminal N1 and a second input terminal N2 respectively connected to a first voltage reference, particularly a supply voltage Vcc, and to a second voltage reference, particularly a ground GND, as well as a third input terminal N3 connected to the second terminal T2, connected in turn to the ground GND.
The driving circuit has also a first output terminal N4 and a second output terminal N5 respectively connected to the gate terminals of the synchronous rectifiers SR1 and SR2, as well as a forth input terminal N6 receiving an external signal SETANT.
Finally, the driving circuit 3 has a fifth input terminal N7 connected to an intermediate terminal of a resistive divider 4, comprising a first resistive element R1 and a second resistive element R2, inserted between the first terminal T1 of the winding 2 and the ground GND, as well as a sixth input terminal N8 connected, by means of a third resistive element R3 to the first terminal T1 of the winding 2.
In particular, the fifth input terminal N7 receives an internal clock signal, while the sixth input terminal N8 receives an inhibition signal INHIBIT.
A driving circuit 3 for the synchronous rectifiers on the secondary side of insulated topologies (particularly of the Forward type), having a configuration like the one shown in FIG. 2, is marketed by STMicroelectronics under the product name STSR2 and it is described in the above-mentioned European patent application. Nevertheless, other synchronous rectifier driving circuits realized according to the prior art can be used.
It can be easily verified that, when the device 1 operates in the discontinuous mode, the synchronous rectifier SR2, operating as free-wheeling for the inductor L1 current, is turned on and off several times in the same switching period, thus causing an increase in switching losses, as well as generating irradiated noise.
The same problem occurs when the synchronous rectifiers SR1 and SR2 are driven by integrated circuits usually called drivers, which, upon detecting the switching transition by means of a threshold comparator, generate convenient output signals for the synchronous rectifiers SR1 and SR2. In this case, the trend of the driving signal outputted by these drivers is however not correct, as graphically shown in FIG. 3, showing the trend of the voltage signal (signal A) applied to the free-wheeling synchronous rectifier SR2 in correspondence with switchings detected (signal B) by the threshold comparator.
Although it is possible to change the threshold value of the comparator used to detect switching transitions, in many cases the oscillation of the comparator input signal has actually an amplitude comparable to the amplitude of the squared signal causing the incorrect driving of the synchronous rectifiers.
The device 1 shown in FIG. 2 is capable of providing two driving signals with a dead time DT set so as to correctly drive the synchronous rectifiers, as schematically shown in FIG. 4.
In particular, the driving circuit 3 comprises a peak detector connected to the input of the clock signal Ck. This detector is able to distinguish the switching transitions of the primary MOS transistor and possible sinusoidal wave signals caused by the discontinuous mode operation. The signal INHIBIT is used to turn the free-wheeling synchronous rectifier SR2 off during the dead time DT. An embodiment of a driving circuit 3 of this type is schematically shown in FIG. 2A.
In particular, the driving circuit 3 shown in FIG. 2A comprises a gate control circuit 5 input-connected to the fifth N7 and sixth input terminal N8, by means of a first 6 and second threshold comparator 7 respectively. The gate control circuit 5 is also output-connected to the first N4 and second output terminal N5, by means of a first 8 and second output buffer 9 respectively.
Actually, the peak detector is able to perform a correct distinction only when a difference ΔV of at least 400 mV is present between the squared signal and the sinusoidal signal.
In fact, when the square wave signal and the sinusoidal signal have comparable amplitudes, if not the same, and even more when the sinusoidal signal has a higher amplitude value than the square wave signal, the peak detector of the driving circuit 3 is not able to correctly distinguish switching transitions of the primary winding.
This known solution, although advantageous under several aspects, cannot however be used in all applications.
A need accordingly exists in the art to provide a synchronous rectifier driving circuit able to correctly detect switching transitions even when the conversion device associated thereto operates in the discontinuous mode, thus overcoming the limits and drawbacks still affecting prior art devices.